1. Field of the Invention
The present invention relates to a backlight unit, and more particularly to a backlight unit with a lamp holder.
2. Description of the Related Art
Generally, liquid crystal display devices (LCDs) are increasingly being used because they are light, thin and consume low power. For example, liquid crystal display devices are used in office automation equipment, audio/video equipment, and so on. In a liquid crystal display device, light transmission is controlled in accordance with video signals applied to a plurality of control switches arranged in a matrix, thereby displaying a desired picture on a screen.
The LCD is not a self-luminous display device. It requires a light source, such as a backlight. There are two types of backlight for the LCD device: a direct type backlight and an edge type backlight. The edge type backlight, which is used in medium to small size liquid crystal display panels, has a lamp installed at an outer area of a flat panel to irradiate light in such a way that the light generated from the lamp is incident on the entire surface of the liquid crystal display panel through a transparent light guide panel. The direct type backlight, which is used in medium to large size liquid crystal display panels, has a plurality of lamps arranged in a plane, and a diffusion plate installed between the lamp and a liquid crystal display panel in order to irradiate a light onto a large and middle sized liquid crystal display panel.
FIG. 1 is a perspective view of a liquid crystal display device with a direct type backlight according to the related art. Referring to FIG. 1, the direct type liquid crystal display device includes a liquid crystal display panel 2 and a backlight unit which irradiates light onto the liquid crystal display panel 2. The liquid crystal display panel 2 has liquid crystal cells forming an active matrix between upper and lower glass substrates, and a thin film transistor installed for switching a video signal to each of the liquid crystal cells. A refractive index of each liquid crystal cell is changed in accordance with the video signal, thereby displaying a corresponding picture. A tape carrier package TCP (not shown) is stuck onto a lower substrate of the liquid crystal display panel 2. A driver integrated circuit IC is mounted on the TCP to apply a drive signal to the thin film transistor. Further, polarizing sheets 8 and 18 are each installed in front and rear surfaces of the liquid crystal display panel 2. The polarizing sheets 8 and 18 improve the viewing angle of a displayed picture.
The backlight unit includes a plurality of lamps 36. The lamps 36 receive power from an external power source, irradiate light onto the liquid crystal display panel 2, and are independently driven. A case 34 is provided to hold the lamps 36. A reflection plate 14 is installed between the lamps 36 and the case 34. The reflection plate 14 reflects light generated by the lamps 36, thereby preventing light leakage. A diffusion panel 12 is provided to diffuse light generated from the lamp 36 or reflected by the reflection plate 14 toward the liquid crystal display panel 2. A plurality of optical sheets 10 distributes the light diffused from the diffusion panel 12 onto the liquid crystal display panel 2, thereby improving the light efficiency.
Each of the lamps 36 includes a glass tube, an inert gas inside the glass tube, a cathode and an anode which are installed at both ends of the glass tube. The inert gas is charged in the inside of the glass tube, and a phosphorus is spread over the inner wall of the glass tube. In each of the lamps 36, an inverter applies an AC voltage to a high voltage electrode and a low voltage electrode. Electrons radiate from the low voltage electrode and collide with the inert gas inside the glass tube. The numbers of electrons increase according to a geometric progression. Then, the excess electrons cause an electric current to flow inside the glass tube, and excite the inert gas, which emits an ultraviolet ray. The emitted ultraviolet ray collides with luminous phosphorus spread over the inner wall of the glass tube, thereby radiating a visible ray.
A fixed gap is maintained between the lamps 36 in the holding case 34. The reflection plate 14 is arranged between an upper surface of the case 34 and the lamps 36, reflects the light generated from the lamps 36, and irradiates the generated light toward the liquid crystal display panel 2, thereby improving light efficiency. A diffusion pattern formed on the diffusion panel 12 diffuses the light generated from the lamps 36 or reflected by the reflection plate 14. The diffusion pattern causes the incident light to propagate toward the liquid crystal display panel 2 with a wide angle.
The optical sheets 10 increase the brightness of the diffused light from the diffusion panel 12, thereby improving the brightness of the liquid crystal display device. Further, light slantingly incident from the surface of the diffusion panel 12 and the reflection sheet 14 is vertically oriented by the optical sheets 10 toward the liquid crystal display panel 2. Thus, the optical sheets 10 act set the propagation direction of the light exiting from the surface of the optical sheets 10.
In the related art liquid crystal display device, a fixed distance is provided between the lamps 36. Moreover, the lamps 36 are installed at a fixed distance from the surface of the case 34. Accordingly, the related art liquid crystal device provides an uniform display brightness, thereby reducing visual fatigue as compared to a cathode ray tube CRT. On the other hand, experiments show that, for a user watching the screen, the focus of attention is the central portion of the screen. For example, with the amount of information transmitted to the user in mid to large size TV screen, the user focuses his attention on the central portion of the screen rather than on the edge. Thus, it is beneficial to emphasize brightness in the central portion of the screen. However, the arrangement of the lamps 36 in the related direct type backlight does not provide brightness enhancement in the central portion of the screen. The related art direct type backlight irradiates light with the same brightness toward the edges of the screen as in the center of the display. Thus, light from the related art direct type backlight is unnecessarily bright at the edges. Accordingly, power is wasted unnecessarily.